1. Field of the Invention
The present invention relates to an electrophotographic or electrostatic image forming apparatus. In particular, the present invention relates to an image forming apparatus which forms a visible image on an image-holding member and transfers the visible image onto a transfer-receiving medium (or a printing sheet, hereinafter referred to as a "transfer medium"). The image forming apparatus is preferably realized in a multicolor or unicolor electrophotographic copying machine, and a laser beam printer.
2. Related Background Art
As an example of conventional image forming apparatuses, a color laser beam printer is explained. The color laser beam printer has a drum-shaped electrophotographic photosensitive member, namely a photosensitive drum, held rotatably in the center portion of the apparatus and rotating in one direction. Around the photosensitive drum, there are provided a primary electrifier for charging electrically the photosensitive drum uniformly, a laser beam light exposing means for projecting image information to form a latent image, a plurality of developing devices for developing latent images to form visible images (toner images), and an intermediate transfer belt as an intermediate transfer member for receiving the visible images from the photosensitive drum. The toner image formed on the photosensitive d rum is transferred onto the surface of the intermediate transfer belt. The toner remaining on the photosensitive drum is removed by a cleaning device for use of the photosensitive drum in the subsequent image formation process.
By repetition of the above process, the color toner images are transferred in superimposition on the intermediate transfer belt. Thereafter, the multicolor toner image is transferred from the intermediate transfer belt onto a transfer medium fed from a feeding device. The transfer medium having received the multicolor image is delivered by a delivery belt to a fixing device, where the toner image is fixed on the transfer medium by heating and pressing with a fixing roller and a pressure roller. The transfer medium having the toner image fixed thereon is discharged out of the apparatus.
Such an image forming apparatus has a plurality of detectors, namely jam sensors, for detecting the transfer medium to confirm the reliable feed, delivery, and discharge of the transfer medium which are provided on the paths of feed, delivery, and discharge. Another detector for transfer medium is provided also just behind the fixing roller and the pressure roller (in the vicinity on the downstream side thereof) to detect the winding of the transfer medium onto the rollers.
The detector for the transfer medium has a flag which is energized in one direction by a spring or the like and is swung only on passage of the transfer medium, and a photosensor which is turned on and off by means of the flag.
The above conventional apparatus, however, may have disadvantages described below owing to direct rubbing of the transfer medium with the flag on passage of the transfer medium through the transfer medium detecting means.
When the transfer medium is an OHT, it is easily scratched in stripes on its portions rubbed with the flag because the OHT is very liable to be scratched.
When images are formed on both faces of the transfer medium, the flag slides necessarily on the surface of the image having been formed on the first face at the time of forming an image on the second face, thus scratching the image in stripes or scraping the image on the first face of the transfer medium.
Further, the portion of the flag coming in contact with the transfer medium may be worn out, and the amount of the contact may become reduced.